The present invention relates to methods and apparatus for bonding leads of an electrical device, and more particularly to a method and apparatus for bonding external leads of an integrated circuit or similar device to effect electrical and physical connections.
A commonly known type of physically and electrically bonding an electrical or electronic component to an electrical circuit is referred to as soldering. In soldering, a molten metal, such as a tin-lead alloy, is applied to a junction between a component and the circuit it is to be bonded to. When the molten metal at this junction solidifies by cooling, the component is fused and bonded to the circuit.
There are numerous methods of melting and applying the solder to the component. One method is to prepare a bath of molten solder and then dip the leads to be soldered into this bath. A special version of this method is called wave soldering and is used on through-hole printed circuit boards. This is an excellent method, but it is limited to the soldering of larger components with significant lead-to-lead separation. Unfortunately for the more modern high density components, e.g. lead pitches of 0.020 inches, solder bath techniques not only solder the component lead to the desired conductor, but also bridge over to make undesired connections with adjacent leads and adjacent conductors.
Another method is to apply heat by means of a soldering iron to the junction of a component lead, its connection conductor and some solid solder in order to melt the solder and form the desired bond. One such soldering iron is described in U.S. Pat. No. 2,501,616 entitled Soldering Iron Tip and issued to P. Robinson. This patent explains that some of the usual problems of a soldering iron are that the soldering surface degrades and changes character-istics with use, with the result being poor and inconsistent solder bonds.
An updated version of the soldering iron is called a hot bar. A hot bar may have various geometries in order to solder all leads on at least one side at once. Some hot bar machines solder all leads of an integrated circuit in one operation. The hot bar is precisely placed over the leads of an integrated circuit and then lowered into contact with all of the leads at once. The temperature and amount of heat transferred to the leads are carefully controlled to obtain consistent results.
Unfortunately, the electrical currents that are passed through each hot bar thermode to raise the temperature to the proper level induces a local electric field of a level that can be damaging to integrated circuitry.
The surface of the hot bar, like the soldering iron, also degrades and changes with use. The soldering surface of the hot bar becomes scratched, distorted and worn with use. Distortions and wear are especially troublesome because even and consistent physical contact is necessary for consistent solder bonds. Further, flux residues and dirt will adhere and build up on scratches of the hot bar. This build up not only causes inconsistent heating, but the dirt and scratches also interact with the solder to cause solder bridges to form between the integrated circuit leads, or solder to adhere to both the integrated circuit lead and the hot bar during a cool down cycle, thereby causing a tearing of the solder.
It is an object of the present invention to provide a hot bar soldering surface that is immune to scratches and wear.
It is another object of the present invention to provide a hot bar soldering surface that does not appreciably change its heat transfer characteristics with use.
It is another object of the present invention to provide a hot bar soldering surface that has a high thermal conductance and a low electrical conductance.